When you get a subpoena, you get "use immunity" meaning that nothing you say can be used against you unless you lie (talk to Barry Bonds about that). You can still be prosecuted, but the feds would have to show that everything they used came from a source other than the immunized testimony. That's how Oliver North skated during Ronald Reagan's Iran-Contra scandal - he testified to Congress under a subpoena, and then the feds couldn't show that their evidence against him was untainted. Testifying under a subpoena removes the fifth amendment claim.